Thermal diodes are devices that rectify heat flows. Thermal diodes that have been reported can be divided into two categories, solid-state thermal diodes based on non-uniform nanotubes or two-segment bars, and phase-change thermal diodes based on asymmetric heat pipes, respectively. Although solid-state thermal diodes can work for a wide range of temperature, reported diodicities are very low (typically <1.1). Likewise, heat pipe based thermal diode exhibit high diodicity (typically >100), however, the rectification is dramatically reduced (<10) when one-dimensional heat pipes are painstakingly integrated in a planar configuration. In phase-change thermal diodes, particularly those in a two-dimensional (planar) configuration, it is a tremendous challenge to design a system to ensure effective liquid return from the condenser to the evaporator. The planar thermal diode with phase change is implemented in a closed-system often called a vapor chamber, which is essentially a planar version of the heat pipe.
State-of-the-art vapor chamber heat spreaders typically use porous wick structures with pore sizes of the order of 100 micron or less to induce capillarity-driven liquid return, but such wick structures have distinct and inherent disadvantages. For example, wick structures generally have very large thermal resistance, which inhibits efficient heat transfer. Another disadvantage of wicked vapor chambers is manufacturability, especially when incorporating wick structures on the side walls and integrating them with in-plane wick structures. Orientation dependence is also an issue for conventional vapor chambers because gravity can affect the return of liquid from the evaporator to the condenser. It is therefore advantageous is to have a wickless evaporator that does not need any porous wicking structure at all, and to have a liquid return mechanism that does not depend on the orientation of external forces.